1. Field of the Invention
This invention relates generally to glass compositions and glass articles and, more particularly, to methods of making highly transparent glass having an aesthetically pleasing blue edge coloration using conventional float glass systems and to the resultant glass articles.
2. Technical Considerations
U.S. Pat. No. 5,030,594 discloses an aesthetically pleasing glass composition useful, for example, in furniture applications such as tabletops or shelving. This glass is highly transparent when viewed normal to a major surface of the glass but has an aesthetically desirable blue (i.e., azure) edge color when viewed on edge. This known glass composition can be made in a multi-stage melting and vacuum-assisted refining operation as disclosed in U.S. Pat. No. 4,792,536. The refining stage of this known process is performed under a vacuum to reduce the concentration of dissolved gasses and volatile gaseous components, particularly sulfur-containing components. As will be appreciated by one skilled in the art, it can be advantageous to remove sulfur-containing components from certain float glass compositions since the combination of sulfur with iron in the glass can result in amber coloration of the glass at high redox ratios, for example, iron redox ratios above 0.4, especially above 0.5, due to the formation of ferric sulfide (also conventionally referred to as iron sulfide or iron polysulfide). Ferric sulfide can form throughout the bulk glass or in streaks or layers of a glass sheet. As used herein, the term “bulk glass” means the internal portion of a glass piece, such as a glass sheet, that is not chemically altered in the process of forming the glass. For a 2 mm or thicker glass sheet made by a float glass process, the bulk glass does not include the outer region of the glass adjacent the glass surface, for example the outer 25 microns (as measured from the glass surface). The elimination of gaseous sulfur components in the vacuum refining stage of this known process helps prevent the formation of ferric sulfide in the glass and, thus, helps prevent amber coloration.
The glass disclosed in U.S. Pat. No. 5,030,594 has received favorable acceptance in the glass market due at least in part to its high visible light transmittance coupled with its aesthetically pleasing blue edge color. Glass made by the process of U.S. Pat. No. 4,792,536 is also characterized by its uniformity of color through the thickness of a glass sheet due to the absence of amber surfaces. While glass manufacturers may wish to produce glass similar to that disclosed in U.S. Pat. No. 5,030,594, most conventional glass manufacturers do not have access to the multi-stage, vacuum-assisted glass manufacturing system currently used to produce this glass. Most commercial flat glass manufactured today is made using conventional non-vacuum float glass processes which lack the specialized vacuum stage used in U.S. Pat. No. 4,792,536. Moreover, it would not be economically feasible to modify a conventional float glass system to add such a vacuum stage since the costs involved would likely be much larger than the financial return obtained in making this specialized blue edge colored glass.
In a float glass process, molten glass flows from a furnace onto a pool of molten tin in a float bath to form a float glass ribbon. During the float process, oxygen from the bottom surface of the float glass ribbon, i.e., the surface of the ribbon in contact with the molten tin, can diffuse into the molten tin. Consequently, multivalent ions at the bottom surface of the glass can become chemically reduced. For example, sulfur in or near the bottom surface of the glass can be reduced from S+6 (hexavalent sulfur) to S−2 (sulfide). These sulfides can react with iron, particularly ferric iron (Fe+3), to form iron polysulfides at the bottom surface of the glass ribbon. The iron can already be present in the glass or, in some instances, iron present in the molten tin can diffuse into the bottom surface of the glass to react with the sulfides. Iron polysulfide is a powerful colorant and can produce a region or layer of amber color several microns thick on the bottom of the glass ribbon. Thus, if one were to look through the edge of the resultant glass sheet at an oblique angle, the region of amber coloration on the bottom of the glass can make blue glass appear green or yellowish-green. This perceived color shift of the glass edge at oblique viewing angles is not aesthetically desirable for most applications. The undesirable effect of amber coloration on the bottom surface of the glass can also be present in other tinted glass, such as those having a bulk glass color of green or bluish green.
Therefore, it would be advantageous to provide a method of making glass having similar color characteristics to the glass in U.S. Pat. No. 5,030,594 but using a conventional non-vacuum glass manufacturing process and which reduces or eliminates this greenish edge appearance at oblique viewing angles.